[ ... ] > BUT if i take a snapshot from the system, and want to transfer > it to the external HD, i can not set a parent subvolume, > because there isn't any.
Questions like this are based on incomplete understanding of 'send' and 'receive', and on IRC user "darkling" explained it fairly well: > When you use -c, you're telling the FS that it can expect to > find a sent copy of that subvol on the receiving side, and > that anything shared with it can be sent by reference. OK, so > with -c on its own, you're telling the FS that "all the data > in this subvol already exists on the remote". > So, when you send your subvol, *all* of the subvol's metadata > is sent, and where that metadata refers to an extent that's > shared with the -c subvol, the extent data isn't sent, because > it's known to be on the other end already, and can be shared > directly from there. > OK. So, with -p, there's a "base" subvol. The send subvol and > the -p reference subvol are both snapshots of that base (at > different times). The -p reference subvol, as with -c, is > assumed to be on the remote FS. However, because it's known to > be an earlier version of the same data, you can be more > efficient in the sending by saying "start from the earlier > version, and modify it in this way to get the new version" > So, with -p, not all of the metadata is sent, because you know > you've already got most of it on the remote in the form of the > earlier version. > So -p is "take this thing and apply these differences to it" > and -c is "build this thing from scratch, but you can share > some of the data with these sources" Also here some additional details: http://logs.tvrrug.org.uk/logs/%23btrfs/2016-06-29.html#2016-06-29T22:39:59 The requirement for read-only is because in that way it is pretty sure that the same stuff is on both origin and target volume. It may help to compare with RSYNC: it has to scan both the full origin and target trees, because it cannot be told that there is a parent tree that is the same on origin and target; but with option '--link-dest' it can do something similar to 'send -c'. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
